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University  of  Cincinnati 


-BulletiruNo.  21. 


Nov.  J  902. 


Publications  of  the  University  of  Cincinnati, 

Series  II.  Vol.  II. 


Hybridism  and  the  Germ-Cell. 


M.  R  GUYER,  Ph.  D. 


The  University  Bulletins  are  Issued  Monthly 


Entered  ?,t  the  Post  Office  at  Cincinnati,  Ohio,  as  second-class  matter 


University  of  Cincinnati 

Bulletin  No,  21.  Nov.  1902. 


Publications  of  the  University  of  Cincinnati, 
Series  II.  Vol.  II. 


Hybridism  and  the  Germ-Cell. 


M.  F.  GUYER,  Ph.  D, 


The  University  Bulletins  are  Issued  Monthly 


Entered  at  the  Post  Office  at  Cincinnati,  Ohio,  as  second-class  matter 


$c 


c  < 


Hybridism  and  the  Germ-CelL 

MICHAEL    F.    GUYER. 

The  writer  prepared  a  paper  on  "The  Spermatogenesis  of 
Hybrid  and  of  Normal  Pigeons"  in  May,  1900,  and  placed  it 
with  The  Journal  of  Morphology.  On  account  of  the  temporary 
suspension  of  that  periodical  it  is  deemed  desirable  to  publish 
an  abridged  account  of  the  results  of  the  investigation,*  leaving 
details  to  appear  in  the  original  paper  whenever  The  Journal 
shall  resume  publication.  The  chief  interest  of  the  investiga- 
tion centers  in  the  peculiarities  met  with  in  the  transformation 
of  the  germ-cells  of  hybrid  pigeons,  hence  the  present  paper  will 
emphasize  this  phase  of  the  subject. 

The  germinal  cells  of  the  male  pigeon  are  laid  down  in  a 
great  number  of  delicate  convoluted  tubules,  which  wind  back 
and  forth  throughout  the  interior  of  each  testis  and  make  up  its 
main  bulk.  Near  the  periphery  of  each  tubule  are  the  sperma- 
togonia, or  parent  cells  (Fig.  1,  sg.),  which  through  growth  and 
division  give  rise  to  the  various  generations  of  germ-cells  lying 
inward  toward  the  lumen  of  the  tubule.  The  adult  spermatozoa 
are  formed  through  the  final  transformation  of  the  spermatids, 
or  cells  which  lie  nearest  the  center  of  the  tubule,  the  product 
of  the  last  cell-division.  As  in  many  forms,  the  spermatozoa 
attach  themselves  to  a  supporting  cell  (Fig.  1,  s.)  for  a  period 
before  their  complete  maturation  and  ejection  from  the  testis. 

The  usual  four  phases  or  types  of  the  germinal  cells  are 
recognizable,  viz.:  (1)  spermatogonia  (Fig.  1,  sg.),  a  more  or 
less  regular  layer  of  cells  lying  next  to  the  wall  of  the  tubule, 
each  cell  of  which  through  division  gives  rise  to  two  new  cells. 

*  The  original  paper  is  the  thesis  submitted  by  the  writer  as  a 
candidate  for  the  degree  of  Ph.D.,  at  the  University  of  Chicago.  It 
was  accepted  by  the  faculty  in  March,  1900.  A  few  copies  of  a 
limited  edition  of  the  thesis,  published  by  the  author,  are  available 
for  investigators  who  are  specially  interested  in  the  particular 
prcblems  under  discussion.  Address  the  writer  at  the  University 
of  Cincinnati. 

3 


One  or  both  of  these  may  increase  in  size  and  become  (2)  pri- 
mary spermatocytes  (Fig.  1,  scy.  1),  or  remain  in  the  layer  and 
continue  as  spermatogonia.  The  primary  spermatocytes,  after 
some  interesting  changes,  divide  to  form  (3)  the  secondary  sper- 
matocytes (Fig.  1,  scy.  2),  which  divide  again  shortly  and  give 
rise  to  (4)  the  spermatids  (Fig.  1,  st.),  through  the  transforma- 
tion of  which  the  spermatozoa  are  developed.  The  number  of 
chromosomes  in  each  type  as  seen  at  the  equator  of  the  spindle 
before  division  is,  in  the  spermatogonia  sixteen  loops,  in  primary 
spermatocytes  eight  rings,  and  in  secondary  spermatocytes  four 
rings.  The  nurse  cells,  or  Sertoli  cells,  to  which  the  sperma- 
tozoa become  attached  at  one  period  of  their  transformation 
(Fig.  1,  s.),  are  irregularly  disposed  among  the  other  cells. 

THE   SPERMATOGONIA. 

The  spermatogonia  lie  in  a  more  or  less  regular  layer  along 
the  wall  of  the  tubule.  In  early  stages  they  are  far  apart  and 
possess  small  nuclei,  which  are  oblong  with  the  long  axis  par- 
allel to  the  tubule  wall.  The  cell  boundaries  are  at  first  very 
indistinct  or  seemingly  absent,  and  gaps  frequently  intervene 
between  the  individual  spermatogonia,  so  that  the  latter  seem 
to  have  been  left  behind  from  a  preceding  set,  or  to  have 
recently  settled  in  their  present  position.  In  later  stages  the 
cells  are  crowded  together  till  they  become  columnar  in  shape, 
while  the  nuclei  increase  in  size  and  become  very  distinct.  A 
condensation  of  the  cytoplasm.,  or  mass  of  sphere  substance 
(idiozome  of  Meves),  makes  its  appearance,  and  gradually  in- 
creases in  size  till  it  becomes  a  well  defined  area  (Fig.  2,  i). 
Near  the  center  of  this  mass  is  generally  a  clear  area,  in  which 
a  minute  centrosome  (c)  is  discernible.  A  nucleolar-like  mass 
is  usually  visible  within  the  nucleus,  but  judging  from  its  reac- 
tions to  various  stains,  it  is  nothing  more  than  a  clump  of  the 
same  material  that  composes  the  linin.  This  mass  usually  takes 
part,  together  with  the  linin  network,  in  forming  the  achromatic 
sheath  within  which  each  individual  chromosome  is  incased 
when  ready  for  division.  Before  division  the  nucleus  passes 
through  an  incomplete  spirem  stage.  The  spirem  breaks  up 
into  individual  chromosomes,  which  are  visible  as  irregular 
threads  and  loops  scattered  throughout  the  nucleus.  A  filament 
two  or  three  times  as  long  as  the  other  chromatic  bodies  is  to 


be  seen  at  times.  Before  the  formation  of  the  spindle  for  the 
ensuing  cell  division,  this  body  is  ejected  into  the  sphere  sub- 
stance, where  it  seems  to  ultimately  break  up  and  become  scat- 
tered throughout  the  cytoplasm.  The  significance  of  this  phe- 
nomenon could  not  be  determined.  It  seems  improbable  at 
present  that  the  extruded  body  can  be  homologized  with  the 
''accessory  chromosome"  of  McClung1,  a  curious  nuclear  ele- 
ment, which  he  describes  as  occurring  in  Xiphidium  fasciatum, 
one  of  the  Locustidae.  The  " accessory  chromosome,"  accord- 
ing to  his  account,  does  not  disintegrate  and  disappear,  but 
retains  its  individuality,  and  persists  throughout  the  entire 
period  of  spermatogenesis,  to  take  part  finally  in  the  formation 
of  the  spermatozoon. 

After  the  formation  of  the  chromosomes,  the  centrosome, 
which  lies  in  a  clear  area  of  the  sphere,  divides  into  two,  one  of 
which  moves  along  the  outer  periphery  of  the  nucleus  to  the 
opposite  pole.  The  first  appearance  of  the  spindle  fibers  is  as 
radiations  which  spread  around  the  nucleus  from  the  centro- 
somes.  When  the  mitotic  figure  is  fully  formed,  the  spindle  is 
short  and  broad  and  the  chromosomes  lie  in  a  confused  band  at 
the  equator  (Fig.  3).  The  individual  chromosomes  are  loop- 
shaped,  with  the  closed  end  of  the  loop  toward  the  center  of 
the  spindle.  While  moving  toward  the  poles  after  division,  not 
infrequently  the  free  ends  of  a  chromosome  fuse  to  form  a  small 
ring.  After  repeated  attempts  at  counting,  it  was  determined 
that  sixteen  chromosomes  are  present  at  the  equator  of  the 
spindle  before  division. 

PRIMARY   SPERMATOCYTES. 

The  primary  spermatocytes  originate  from  the  cells  of  the 
last  spermatogonial  division  through  a  process  of  growth.  The 
chromatin  passes  into  the  resting  condition,  and  an  increase  in 
bulk  of  both  the  nucleus  and  the  cytoplasm  begins.  The  sphere 
first  appears  as  an  indistinct  granular  crescentic  area  closely 
applied  to  the  nucleus,  with  the  horns  of  the  crescent  so  ex- 
tended as  to  inclose  more  than  half  of  the  nuclear  surface.  As 
the  young  spermatocyte   grows,  the  sphere  also  increases  in 

1  McClurg,    E.    C.     A   Peculiar   Nuclear   Element   in   the   Male 
Reproductive  Cells  of  Insects.    Zool.  Bui.  II.  4,  1899. 


size,  becoming  more  and  more  rounded.  From  an  early  stage 
a  minute  centrosome  is  visible  in  the  midst  of  the  sphere  sub- 
stance. It  is  surrounded  by  a  clear  area,  which  becomes  more 
pronounced  as  the  sphere  grows  older.  Thus  the  developing 
cell  gradually  acquires  characteristics  of  size,  shape  and  general 
appearance,  that  differ  markedly  from  those  of  the  previous 
generation. 

Synapsis. —  Synapsis  occurs  in  the  primary  spermatocytes, 
during  which  there  is  a  marked  drifting  of  the  chromatin  to  the 
side  of  the  nucleus  in  contact  with  the  sphere  (Fig.  4).  Some 
substance  from  the  nucleus  apparently  passes  out  into  the 
sphere:  it  may  possibly  be  concerned  in  the  formation  of  the 
extremely  coarse-fibered  spindle,  for  almost  immediately  the 
centrosome  divides  and  the  spindle  appears.  In  the  ensuing 
division  of  the  spermatocyte  only  eight  chromosomes  are  pres- 
ent, but  they  are  in  the  form  of  heavy  rings,  and  are  evidently 
bivalent  (Fig.  7). 

During  division  the  eight-ring  chromosomes,  which  are 
incased  in  capsules  of  linin,  break  transversely,  and  as  they 
move  apart  remain  connected  by  threads  of  the  linin  casing. 
These  threads  constitute  the  interzonal  fibers  (Fig.  8,  if).  An 
intermediate  body  is  present  at  the  equator  of  the  interzonal 
fibers  and  marks  out  the  path  of  the  new  cell  walls  (Fig.  8,  ib). 
The  ring  type  of  chromosome  seen  at  this  division  is  formed 
through  the  breaking  up  or  rearrangement  of  the  prominent 
spirem  (Fig.  5),  which  forms  immediately  after  synapsis.  The 
spirem-like  appearance  inside  the  nucleus  disappears  gradually, 
until  by  the  time  the  centrosomes  reach  their  positions  at  oppo- 
site poles  of  the  nucleus,  the  components  of  the  spirem  are  seen 
as  eight  elongated,  irregular  rings  (Fig.  6),  which  consist  of  a 
linin  groundwork,  in  which  are  imbedded  numerous  granules 
and  lumps  of  chromatin.  The  rings  gradually  condense  into 
a  shorter,  heavier  type,  and  the  chromatin  fuses  in  such  a  way 
that  distinct  granules  are  no  longer  visible.  In  a  few  instances 
rings  were  found  to  consist  of  four  more  or  less  spherical, 
densely  staining  areas,  connected  by  lighter  bands  of  linin.  It 
is  possible  that  this  is  comparable  to  the  tetrad  formation  so 
frequently  observed  in  maturation  phenomena  (Fig.  6,  tr). 


SECONDARY   SPERMATOCYTES. 

The  product  of  the  division  just  discussed  consists  of  two 
cells,  each  of  which  is  considerably  smaller  than  the  primary 
spermatocyte,  and  which  never  attains  to  its  volume.  These 
cells  are  the  secondary  spermatocytes.  They  go  into  a  resting 
stage,  which  is  of  very  short  duration.  When  the  secondary 
spermatocyte  is  ready  for  division,  curiously  enough  only  four 
chromosomes  appear  (Fig.  9).  They  are  of  the  same  shape 
and  size  as  those  in  the  division  of  the  primary  spermatocyte. 
In  dividing,  the  chromosomes  each  break  in  such  a  way  that  a 
stringing  out  of  the  sheaths  of  the  chromosomes  gives  rise,  as 
in  the  primary  spermatocytes,  to  a  system  of  interzonal  fibers, 
which,  as  division  proceeds,  constrict  at  the  equator  to  form 
a  large  intermediate  body.  For  some  time  after  division  the 
divided  chromosomes  are  seen  as  four  hollow  vesicles  within 
the  daughter  nuclei.  They  fuse  later,  ordinarily,  into  one  large, 
hollow  sphere  of  chromatin  near  the  center  of  the  nucleus 
(Fig.  10).  Numerous  fine  fragments  of  chromatin  migrate  to 
the  nuclear  membrane,  which  has  appeared  in  the  meantime, 
and  form  a  thin  shell  of  chromatin  along  its  inner  surface.  The 
centrosome  (Fig.  10,  c)  persists,  and  together  with  the  tip  of 
the  spindle  moves  out  into  the  cytoplasm.  The  tip  of  the 
spindle  seemingly  becomes  re-converted  into  sphere  substance. 

THE    SPERMATID   AND    ITS   TRANSFORMATION. 

The  new  cell  formed  from  the  division  of  the  secondary 
spermatocyte  is  the  spermatid,  and  is  the  cell  which  will  ulti- 
mately be  transformed  into  the  spermatozoon.  An  adult  sperma- 
tozoon as  it  exists  in  the  vas  deferens  of  the  pigeon  is  shown  in 
Fig.  14.  The  head  is  long  and  narrow,  and  is  intensely  stained 
by  nuclear  dyes.  Favorable  preparations  show  the  chromatin 
arranged  in  a  series  of  vesicles  within  the  head.  Each  vesicle 
of  this  chain-like  series  incloses  a  clear  area,  which  in  some 
preparations  appears  highly  refractive.  A  remarkable  fact  is 
that  the  number  of  vesicles  is  apparently  the  same  as  the  re- 
duced number  of  univalent  chromosomes  should  be,  namely, 
eight.  In  some  instances,  where  only  six  or  seven  vesicles  were 
present,  it  was  observed  that  one  or  two  were  unusually  large, 
and  hence  probably  equivalent  to  two.     It  will  be  recalled  that 


in  the  secondary  spermatocyte  there  were  only  four  chromo- 
somes, but  that  they  were  of  the  bivalent  type,  or  really  com- 
parable to  eight  ordinary  chromosomes.  There  is  no  positive 
evidence  that  the  vesicles  in  the  head  of  the  spermatozoon  cor- 
respond to  individual  chromosomes,  but  the  striking  coincidence 
in  number  is  at  least  very  suggestive,  and  it  would  not  be  sur- 
prising if  the  fact  develops  later  that  after  the  entrance  of  the 
spermatozoon  into  the  egg  the  vesicles  resolved  themselves  into 
eight  distinct  chromosomes. 

At  the  anterior  end  of  the  head  is  a  slender,  fine-pointed 
head-spine.  The  head  posteriorly  connects  directly  with  the 
long  cytoplasmic  tail.  No  middle  piece  is  visible.  The  tail  and 
the  head-spine  are  very  difficult  to  observe  accurately,  and  but 
little  of  the  details  of  their  structure  could  be  worked  out.  The 
only  way  to  gain  a  satisfactory  knowledge  of  the  spermatozoon 
at  all  is  through  a  study  of  its  development. 

In  the  transformation  of  the  spermatid  to  form  the  sper- 
matozoon the  first  change  to  be  observed  is  in  the  centrosome. 
It  divides,  and  one  of  the  resulting  centrosomes  enlarges  and 
becomes  ring-shaped  (Fig.  n,  c).  The  axial  filament  of  the 
tail  first  appears  as  a  thread  connecting  the  two  centrosomes, 
and  later  continues  backward  through  the  ring-like  centrosome 
and  out  of  the  cell  (Fig.  n,  ax).  The  smaller  centrosome, 
together  with  material  of  cytoplasmic  origin,  finally  comes  to  He 
within  the  nuclear  membrane.  It  may  be  regarded  perhaps  as 
a  middle  piece,  which  becomes  obscured  by  a  covering  of  chro- 
matin, and  consequently  appears  to  be  absent  in  the  adult 
spermatozoon. 

The  long  head  of  the  spermatozoon  is  the  transformed 
nucleus.  In  the  process  of  elongation  only  what  may  be  termed 
the  anterior  and  the  posterior  ends  of  the  nucleus  extend  at 
first,  but  in  a  short  time  the  entire  nucleus  begins  to  narrow. 
At  the  same  time  the  mass  of  chromatin  at  the  center  sprouts 
out  both  anteriorly  and  posteriorly  to  form  a  central,  thread- 
like core  (Fig.  12).  As  the  process  of  elongation  continues,  a 
narrowing  of  the  sides  of  the  nucleus  takes  place  to  some  extent, 
but  when  one  takes  into  account  the  enormous  elongation  that 
occurs,  together  with  the  relatively  slight  diminution  of  the 
transverse  diameter,  it  becomes  evident  that  there  must  be  con- 
siderable increase  in  the  volume  of  the  nucleus.  The  heavy  cen- 
tral chromatic  filament  after  a  time  becomes  arranged  in  a  wavy 


or  spiral  manner.  As  the  transformation  progresses  the  spiral 
design,  although  often  very  irregular,  becomes  more  perceptible. 
A  splitting  of  this  spiral  core  finally  occurs,  and  thereafter  the 
chromatin  exists  as  two  threads  laid  down  in  an  irregular  double 
spiral  (Fig.  13). 

The  elongation  of  the  nucleus  ceases  at  about  the  time  the 
bisection  of  the  central  filament  has  been  accomplished,  and 
the  nucleus  displays  itself  as  an  enormously  long,  sinuous  head, 
which  may  measure  twice  the  length  of  the  head  of  the  adult 
spermatozoon.  A  dense  protoplasmic  mass  encases  it  and 
extends  backward  along  the  axial  filament.  A  shrinkage  of 
the  nucleus  follows,  in  the  course  of  which  the  double  spiral  of 
chromatin  shortens  and  widens  until  the  exact  relationship  of 
the  chromatin  of  the  two  filaments  can  no  longer  be  deter- 
mined. The  final  appearance  is  that  of  a  chain-like  series  of 
vesicles,  as  described  for  the  mature  spermatozoon  (Fig.  14), 
the  clear  area  in  the  center  of  each  vesicle  corresponding  to 
the  openings  between  the  respective  points  of  intersection  of 
the  two  spirals  of  chromatin. 

The  head-spine  originates  from  a  bubble-like  mass  of  mate- 
rial (Fig.  ii',  v)  which  arises  in  the  sphere.  This  bubble  or 
vacuole  moves  slowly  around  the  periphery  of  the  nucleus  until 
it  lies  at  the  pole  opposite  the  point  at  which  the  centrosome, 
which  marks  the  anterior  end  of  the  axial  filament,  will  enter 
the  nucleus. 

the;  gekm-ce)ll,s  op  hybrid  pigeons. 

In  the  pigeon  some  crosses  are  fertile,  others  are  not.  The 
sterile  hybrids  show  a  greater  or  less  degeneration  of  the  germi- 
nal cells.  The  general  rule  seems  to  be  that  the  more  divergent 
the  parent  forms,  the  more  marked  is  the  degeneration  of  the 
germinal  cells.  From  parents  which  differ  widely  in  structure 
or  habits  there  seems  to  be  much  greater  difficulty  in  securing 
female  than  male  offspring.  I  have  been  able  to  obtain  but  one 
female  offspring  of  very  distinct  species  for  microscopical  exam- 
ination, while,  on  the  other  hand,  I  have  had  six  males.  For 
all  of  this  material  I  am  indebted  to  Professor  Whitman.  From 
the  testis  of  the  offspring  of  the  common  ring  dove,  Turtitr 
risorins,  and  the  white  ring  dove,  Columba  alba,  a  large  number 
of   sections   were   made   for   microscopical   study.     These   two 


forms  are  perfectly  fertile  when  crossed,  and  the  fertility  of 
their  offspring  seems  in  no  wise  diminished.  The  germ-cells 
show  some  of  the  same  phenomena  as  those  of  the  sterile  birds, 
though  in  a  less  marked  degree. 

The  common  brown  ring  dove  when  crossed  with  the  white 
ring  dove  produces  hrowm  offspring.  One  member  of  the  re- 
sulting pair  is  usually  a  few  shades  lighter  in  color  than  the 
other.  In  the  next  or  third  generation  there  is  generally  a 
return  to  the  original  colors  of  the  grandparents ;  one  of  the 
young  is  white,  the  other  brown.  There  is  a  marked  tendency 
for  the  white  ones  to  be  female  and  the  brown  ones  male,  this 
being  true,  at  least  of  the  nine  pairs  killed  by  the  writer.  Occa- 
sionally in  the  third  generation  both  of  the  young  are  white  or 
both  brown. 

Of  the  sterile  hybrids,  whether  male  or  female,  the  sexual 
products  were  abnormal.  The  abnormalities  of  male  hybrids 
may  be  classified  conveniently  under  three  heads':  (i)  Abnor- 
malities in  mitosis ;  (2)  abnormalities  in  the  structure  of  the 
spermatozoon  ;  (3)  degeneration  of  the  germinal  cells.  Not  all 
hybrids  show  these  various  irregularities  in  the  same  degree. 
All  three  kinds  of  the  phenomena  just  mentioned  are  observ- 
able in  the  sterile  forms,  but  the  fertile  birds  differ,  for  the 
most  part,  from  normal  pigeons  only  in  the  slightly  irregular 
character  of  the  mitosis. 

The  abnormalities  in  -mitosis  arc  in  the  nature  of  multipolar 
spindles  and  asymmetrical  division  and  distribution  of  the  chro- 
mosomes (Figs.  15-19).  They  are  more  pronounced  in  sterile 
birds,  but  may  be  met  with  in  fertile  hybrids  also.  It  is  a 
curious  fact  that  the  multipolar  spindles  are  confined  largely  to 
the  primary  spermatocytes,  and  one  is  inclined  immediately  to 
associate  the  fact  with  the  pseudo-reduction  or  formation  of 
bivalent  chromosomes,  which  occurs  normally  at  this  stage  of 
spermatogenesis.  Figs.  15-18  show  some  of  the  different  forms 
of  multipolar  spindles.  The  tripolar  types  are  by  far  the  most 
common.  Fig.  15  represents  perhaps  the  most  prevalent  struc- 
ture. It  was  not  unusual  to  observe  two  spindles  in  one  cell, 
as  shown  in  Fig.  16.  When  two  such  spindles  exist  independ- 
ently in  one  cell,  they  may  each  have  a  small  number  of  the 
large  bivalent  ring-form  chromosomes,  or  a  greater  number  of 
small  chromosomes,  which  are  apparently  univalent.  More 
rarely  both  large  and  small  chromosomes  occupy  one  or  both 

10 


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UBHAHY 


of  the  spindles.  The  facts  indicate  that  a  pseudo-reduction  has 
not  occurred,  or  that  it  is  incomplete  or  abnormal.  What  has 
just  been  said  regarding  the  chromatin  arrangement,  where  two 
separate  spindles  occur,  is  equally  applicable  to  the  multipolar 
forms,  only  there  is  generally  more  variation  in  the  size  of  the 
-chromosomes.  In  a  tripolar  type  like  Fig.  15  the  chromosomes 
are  almost  invariably  numerous  and  of  small  size. 

In  a  few  instances  two  nuclei  were  present  in  the  primary 
spermatocyte,  and  it  seems  probable  that  such  cells  give  rise  to 
the  multipolar  spindles,  bearing  an  excessive  amount  of  chro- 
matin, which  are  sometimes  seen.  Fig.  18  shows  a  tripolar 
spindle  in  a  primary  spermatocyte,  where  there  is  much  variation 
in  the  size  of  the  chromosomes. 

An  asymmetrical  distribution  of  chromatin  results,  of 
course,  in  many  cases  where  the  division  is  by  means  of  multi- 
polar spindles,  but  in  addition  to  this  there  is  very  frequently 
an  unequal  division  of  the  chromosomes  themselves.  This 
occurs  as  often  where  the  spindle  is  single  as  in  any  other  case. 
In  dividing,  perhaps  only  one-quarter  of  a  chromosome  will  go 
to  one  pole  and  the  other  three-quarters  to  the  opposite  pole, 
or  the  division  may  be  such  that  a  portion  of  the  chromatin  is 
cut  out  entirely  and  left  behind  in  the  cytoplasm.  Fig.  19  rep- 
resents a  secondary  spermatocyte,  of  which  one  chromosome 
is  very  minute,  as  if  part  of  its  material  had  been  lost  in  the 
preceding  division. 

In  two  or  three  instances  one  of  the  large  chromosomes  of 
the  primary  spermatocyte  was  observed  to  be  made  up  of  four 
small  rings.  Whether  this  indicates  an  exaggerated  demarca- 
tion into  tetrads  it  is  impossible  to  affirm,  though  the  fact  is  a 
significant  one. 

Abnormalities  in  the  structure  of  the  spermatozoa  are  present 
in  sterile  hybrids.  There  is  a  curious  varicosity  about  the 
middle  of  the  spermatozoon  head  in  such  forms  that  attracts 
the  attention  immediately,  when  the  objects  are  examined  under 
the  microscope  (Fig.  20).  This  enlargement  seems  to  be  almost 
universal  amongst  the  spermatozoa,  and  is  sufficient  of  itself  to 
produce  sterility,  for  such  a  malformation  would  prevent  its 
possessor  from  entering  the  egg.  In  a  very  few  instances  what 
appears  to  be  a  normal  spermatozoon  may  be  observed  among 
the  misshapen  ones,  and  it  is  possible  that  if  these  reached  a 

11 


suitable  egg,  fertility  might  result.  Their  chance  of  meeting:; 
with  the  egg,  however,  is  very  slight. 

A  study  of  the  development  of  these  deformed  spermatozoa- 
reveals  the  fact  that  the  bead-like  enlargement  results  from  the 
incomplete  development  of  the  nucleus  in  the  formation  of  the 
head.  The  two  ends  of  the  nucleus  sprout  out,  as  it  were,  and 
grow  for  a  short  distance,  but  the  remainder  of  the  nuclear-wall 
retains  its  original  form  and  position.  The  arrangement  of  the 
chromatin  is  very  irregular.  A  deeply  staining  mass  is  visible 
in  the  bulb-like  swelling,  from  which  thick  filaments  spring  out 
forward  and  to  the  rear. 

Degeneration  of  the  germinal  cells  was  in  progress  in  the  testes 
of  all  sterile  forms,  but  was  most  pronounced  in  hybrids  from 
birds  which  were  of  very  divergent  species,  or  hybrids  which, 
were  themselves  descendants  of  fertile  hybrids.  There  were 
some  cases  of  such  extreme  degeneration  that  only  the  layer  of 
cells  lying  along  the  wall  remained  in  the  tubule.  Where  such 
a  degree  of  degeneration  exists  there  is  of  course  no  approach 
to  the  formation  of  spermatozoa.  There  is  often  a  strong  inva- 
sion of  wandering  cells  into  the  tubules,  especially  where  the 
degenerative  activities  have  become  extensive.  The  inter- 
spaces between  such  tubules  are  also  usually  packed  with  cells 
which  resemble  white  blood  corpuscles.  In  some  preparations 
it  looks  really  as  if  the  germinal  cells  themselves  lose  their  walls 
and  characteristic  appearance  and  become  leucocytes,  though 
no  definite  conclusions  could  be  reached  regarding  this  point. 
Some  tubules  are  occupied  almost  entirely  by  cells  which  have 
the  exact  appearance  of  the  large  stroma  cells  present  outside 
the  tubules. 

The  primary  spermatocytes  seem  to  be  the  cells  most  sus- 
ceptible to  decay.  Frequently  the  nuclear  contents  have  a 
watery  and  disintegrated  appearance  and  the  sphere  substance 
is  marked  by  the  presence  of  a  large  vacuole  in  its  center.  In 
testes  where  degenerative  processes  are  pronounced  a  number 
of  cells  may  run  together  to  form  a  giant  cell,  in  the  center 
of  which  is  an  enormous  vacuole  surrounded  by  the  nuclei 
of  the  original  cells.  The  mass  thus  formed  is  very  similar  in 
appearance  to  the  giant  cells  found  in  many  pathological  tissues. 

A  detailed  description  of  the  individual  hybrids  will  not  be 
entered  into  at  present,  but  will  be  left  to  the  original  paper. 
The  crosses  were :     (i)  Male  black  tumbler,  female  brown  ring 

12 


dove ;  (2)  male  black  tumbler,  female  brown  ring  dove ;  (3) 
male  hybrid  from  white  and  brown  ring  dove,  female  homer; 
(4)  male  wild  passenger  pigeon,  female  brown  ring  dove;  (5) 
male  wild  passenger  pigeon,  female  brown  ring  dove ;  (6)  male 
turtle  doYQ,  female  Japanese  turtle  dove.  The  hybrid  from  the 
last  named  was  a  female.  The  ovary  and  oviduct  were  rudi- 
mentary. Occasional  small  ova  with  an  incomplete  follicle  were 
present.  The  largest  egg  had  attained  to  a  diameter  of  only 
seventy-five  micra ;  (7)  numerous  crosses  of  white  and  brown 
ring  doves. 

CONCLUSIONS    FROM   TH£    STUDY    OF   HYBRIDS. 

It  appears  that  in  crosses  of  very  divergent  species  the 
degenerative  processes  in  the  germ-cells  are  at  a  maximum. 
In  closely  related  forms  like  the  brown  and  the  white  ring 
doves  fertility  is  not  diminished,  and  the  testes  seem  to  be 
normal,  except  for  occasional  irregularities  in  mitoses. 

The  formation  of  multipolar  spindles  in  division  and  the 
unequal  distribution  of  chromosomes  seen  in  many  instances 
are  among  the  most  interesting  phenomena  presented.  Such 
irregularities,  however,  occur  to  a  slight  extent  in  normal  birds. 
A  very  careful  study  reveals  the  fact  that  they  are  more  preva- 
lent in  the  ordinary  dove-cot  pigeons  than  in  pure  breeds  of 
doves,  which  have  bred  true  for  many  generations. 

That  the  irregularities  of  division  in  hybrids  are  due  to 
degenerative  processes  going  on  in  the  tubules,  which  give  rise 
to  deleterious  chemical  substances,  is  the  first  thought  that  pre- 
sents itself.  This  seems  a  very  acceptable  idea  from  what  we 
know  oi  the  effects  of  drugs  upon  cell  division,  but  it  does  not 
account  for  the  fact  that  the  primary  spermatocytes  are  the 
cells  attacked  in  the  great  majority  of  cases. 

A  hybrid  offspring  is  really  a  compound  of  two  very  dif- 
ferent individual  plasmas,  hence  conflicting  tendencies  must  nec- 
essarily have  been  induced  within  its  body.  The  abnormality 
in  division  may  be  but  an  attempt  of  each  plasma  to  assert  its 
individual  activity.  But  why  does  this  effort  become  apparent 
only  in  the  primary  spermatocytes?  The  answer  appears  to  be 
simply  that  there  is  no  necessity  for  fusion  of  the  components 
of  the  chromatin  of  cells  except  in  the  germ-cells  at  one  stage 
of  their  maturation.     Investigators  have  found  that  in  matura- 

13 


tion  of  germ-cells  a  reduction  of  the  ordinary  number  of  chro- 
mosomes to  one-half  occurs.  Before  this  actual  reduction  there 
is  ordinarily  a  so-called  pseudo-reduction,  in  which  the  chromo- 
somes fuse  in  pairs,  so  that  when  the  cell  is  ready  for  division, 
although  only  half  of  the  regular  number  of  chromosomes 
appear,  each  is  really  double  (bivalent),  and  equivalent  to  two 
of  the  simple  (univalent'!  type.  In  spermatogenesis  this  change 
generally  comes  about  in  the  primary  spermatocyte.  Now,  in 
hybrids  it  may  be  supposed  that  in  the  ordinary  cells  of  the  body 
the  chromosomes  from  the  paternal  and  the  maternal  species 
lie  side  bv  side  and  carry  on  the  customary  functions  of  the 
cells,  but  when  it  comes  to  an  actual  fusion  of  chromosomes 
to  form  the  bivalent  type  necessary  for  reduction,  the  incom- 
patibility of  the  two  different  plasmas  renders  the  union  incom- 
plete or  prevents  it  entirely.  That  the  ordinary  somatic  cells  of 
hybrids,  either  plants  or  animals,  are  under  the  influence  of  two 
distinct  tendencies  is  well  shown  in  the  decided  mosaic-like 
structures  which  frequently  occur,  the  classic  figure  being  to 
liken  hvbrids  to  warp  and  woof.  To  cite  but  one  example, 
Macfarlane  found  that  a  hybrid  of  the  gooseberry  and  black 
currant,  instead  of  being  a  strictly  intermediate  type,  really  pos- 
sessed, side  by  side/organs  characteristic  of  each  parent;  the 
leaves  bore  both  the  shield-shaped,  oil-secreting  hairs  of  the 
currant  and  the  simple  hairs  of  the  gooseberry,  though  each 
hair  was  but  half  the  size  of  the  parent  type.  We  may  infer,  then, 
that  in  certain  hybrid  pigeons  the  univalent  chromosomes  from 
each  of  the  parents  may  lie  side  by  side  in  the  ordinary  cells  of 
the  body  and  divide  normally,  but  when  it  comes  to  the  period 
of  fusion  in  the  germ-cell,  they  will  not  unite  to  form  the  biva- 
lent type,  or  else  they  unite  incompletely.  The  result  is  that  in 
the  primary  spermatocyte,  instead  of  one  spindle  bearing  eight 
bivalent  chromosomes,  a  multipolar  spindle,  or  not  infrequently 
two  separate  spindles,  bearing  two  groups  of  univalent  chro- 
mosomes, may  appear.  In  cases  where  both  large  and  small 
chromosomes  are  seen,  it  is  necessary  to  suppose  that  a  loose 
union  has  occurred  in  some  chromosomes.  The  unequal  divis- 
ions of  the  bivalent  chromosomes  of  many  hybrids  indicate  that 
such  chromosomes  have  in  some  way  been  rendered  very 
unstable. 

If  we  accept  the  view  that  chromatin  is  a  substance  capable 
of  varying  in  qualities  in  the  different  regions  of  the  chromo- 

14 


some,  then  in  fertile  hybrids,  where  irregular  mitoses  occur,  the 
different  germ-cells  will  certainly  not  be  qualitatively  similar 
after  division,  and  one  would  expect,  the  offspring  produced 
from  such  cells  to  be  variable.  That  the  chromatin  of  each 
parent  species  often  retains  its  individuality  is  indicated  by  the 
fact  observed  in  many  primary  spermatocytes  where  two  sep- 
arate groups  of  the  small  or  single  type  of  chromosome  exists. 
The  division  of  such  a  cell  into  three  or  four,  as  the  case  may 
be,  results  in  the  formation  of  new  cells,  some  of  which  will 
manifestly  contain  chromatin  from  only  one  of  the  original 
parent  species,  and  some  only  from  the  other.  Some  of  the 
spermatozoa,  then,  will  bear  chromatin  from  only  one  of  these 
species.  In  the  offspring  from  such  a  cell  one  would  expect  a 
much  closer  return  to  whichever  one  of  the  parent  forms  it 
represented  than  in  the  offspring  of  a  "mixed"  spermatozoon. 
In  discussing  irregular  divisions,  however,  it  must  not  be 
forgotten  that  many  apparently  normal  divisions  of  the  primary 
spermatocytes  also  occur  in  all  hybrids,  and  constitute  by  far 
the  predominant  kind  of  division  in  hybrids  from  closely  related 
forms.  Unequal  distributions  of  chromatin  cannot  there- 
fore play  the  most  important  part  in  variation  or  rever- 
sion. There  seems  to  be  no  other  interpretation,  indeed,  than 
that  in  the  many  normal  mitoses  of  the  bivalent  chromosomes 
which  occur,  the  chromatin  of  the  father  and  of  the  mother  is 
set  apart  so  that  the  ultimate  germ-cells  are  what  might  be 
termed  "pure"  cells;  that  is,  a  given  egg  or  sperm-cell  con- 
tains exclusively  or  at  least  predominantly  qualities  from  one 
parent.  The  offspring  from  fertile  hybrids  of  the  same  parent- 
age might  then  be  similar  to  the  mixed  type  of  the  original 
hybrid,  or  revert  to  one  of  the  grandparent  types,  dependent 
upon  the  chances  of  the  various  cells  for  union  at  fertilization. 
Tf  a  spermatozoon  and  an  egg  containing  characteristics  of  the 
same  species  unite,  then  the  reversion  will  be  to  that  species ; 
if  a  sperm-cell  containing  the  characteristics  of  one  species  hap- 
pens to  unite  with  an  ovum  containing  characteristics  of  the 
other  species,  then  the  offspring  will  be  of  the  mixed  type  again. 
By  the  law  of  probability  the  latter  will  be  the  more  prevalent 
occurrence,  because  there  are  four  combinations  possible,  and 
two  of  the  four  would  result  in  the  production  of  mixed  off- 
spring, while  only  one  combination  could  result  in  a  return 
to  one  of  the  ancestral  species. 

15 


From  the  fact  that  in  cases  of  apparently  complete  return  to 
one  parent  type,  characteristics  of  the  other  parent  may  never- 
theless crop  out  from  time  to  time  in  succeeding  generations, 
it  is  evident  that  all  ot  the  germ- cells  are  not  absolutely  "pure." 
The  occasional  inequalities  in  the  division  of  individual  chromo- 
somes, as  already  mentioned,  may  account  for  this  fact.  It  is 
probable  that  the  irregular  distributions  of  chromatin,  where 
such  occur,  have  more  to  do  with  such  succeeding  offspring  as 
show  variation,  and  less  with  those  which  return  to  the  specific 
types.  In  the  latter  case  the  chromatin  of  each  species  has 
remained  entirely  distinct  (in  marked  hybrids),  or  has  normally 
separated  again  at  the  sundering  of  the  bivalent  chromosomes 
(in  mild  crosses)  into  the  two  original  plasmas.  It  is  very 
obvious,  of  course,  that  most  of  the  variation  seen  in  the  off-, 
spring  of  fertile  hybrids  is  due  to  the  union  again  of  two  "pure" 
germ-cells,  each  of  which  represents  a  different  one  of  the 
original  parent  species. 

That  irregular  divisions  can  not  account  entirely  for  rever- 
sions to  grandparent  types  is  very  evident  in  the  crosses  of 
brown  and  white  doves,  where  the  irregularities  are  by  far  too 
few  to  equal  the  percentage  of  reversions.  There  is  but  one 
ultimate  conclusion,  then,  namely,  that  the  irregularity  in  divis- 
ion of  the  primary  spermatocytes,  which  appears  in  hybrids 
between  very  different  species,  is  but  an  index  to  what  occurs 
in  ordinary  crosses.  In  the  latter,  instead  of  separate  spindles 
and  non-fusion  of  chromosomes,  a  true  union  occurs,  but  the 
bivalent  chromosomes  ultimately  divide  in  such  a  way  that  the 
respective  plasmas  occupy  different  cells.  There  is  a  separation 
of  the  paternal  and  the  maternal  chromosomes  which  had  fused 
during  synapsis. 

With  regard  to  the  question  of  the  persistence  of  chromo- 
somes, the  evidence  is  becoming  stronger  every  day  that  these 
elements  do  retain  their  individuality.  Riickert,2  for  instance, 
in  his  study  upon  the  fertilization  of  cyclops,  was  able  to  follow 
the  maternal  and  paternal  chromosomes  very  distinctly  in  cleav- 
age. Again,  to  cite  only  one  or  more  of  the  rapidly  multiplying 
examples,  Herla3  and  Zoja4  have  shown  that  in  the  hybrid  fer- 

2  Riickert,  J.    Zur  Eireifung  bei  Copepoden:    Qu.   Hefte,   1S94. 

3  Herla,   V.     Etude  des  variations   de  la  mitose   chez   l'ascaride 
megalocephala:    Arch.  Biol.,  XIII. ,  1893. 

4  Zoja.  R.    Sullo  independen>.a  della  cromatina  paterna  e  materna 
nel  nucleo  delle  cellule  embrionali:    Anat.  An.  XL,  1895. 

16 


tilization  of  Ascaris,  if  the  eggs  of  variety  bivalens  is  fertilized 
with  the  spermatozoon  of  variety  univalens,  the  three  chromo- 
somes trms  brought  together  retain  their  individuality  and  re- 
appear at  each  cleavage,  at  least  to  the  twelve-cell  stage.  Zoja 
affirms  that  the  paternal  chromosome  is  of  smaller  size  and  is 
thus  distinguishable  from  the  two  maternal  chromosomes. 

The  above  interpretations  are  offered  with  the  hope  that 
they  may  perhaps  lead  to  some  clew  concerning  the  real  nature 
of  the  material  basis  of  heredity.  If  the  conception  proves  to 
be  a  true  one,  then  it  doubtless  affords  a  key,  among  other  prob- 
lems, to  the  long-standing  one  as  to  why  many  plants  will  come 
true  from  slips  or  grafts,  but  not  from  seed.  The  reason  may 
be  sought  in  the  pseudo-reduction  period  of  the  germ-cell. 
Plants  such  as  the  apple,  for  example,  which  do  not  come  true 
from  seed,  are  practically  multi-hybrid.  In  the  germ-cells  there 
will  be  numerous  incompatibilities  due  to  the  fact  that  the  plant 
has  been  miscellaneously  fertilized  for  a  number  of  generations. 
In  propagation  by  means  of  slips,  the  chromosomes  lie  side  by 
side  and  divide  in  the  ordinary  way  to  construct  and  maintain 
the  new  body,  so  that  it  is  practically  a  continuation  of  the  old 
one ;  but  when  the  time  comes  for  maturation  of  the  germ-cells, 
the  lack  of  harmony  between  the  various  plasmas  represented 
asserts  itself,  with  the  result  that  bivalent  chromosomes  are 
formed,  which  divide  in  such  a  manner  as  to  segregate  different 
sets  of  ancestral  qualities.  The  resulting  combinations  in  fer- 
tilization will  give  rise  to  seed  many  of  which  may  possess 
dissimilar  sets  of  qualities. 

As  to  the  other  abnormalities  met  with  in  the  spermato- 
genesis of  hybrids,  about  all  that  can  be  said  is  that  the  whole 
phenomena  show  lack  of  vigor  in  the  development  of  the  germ- 
cells,  whatever  this  may  mean.  The  deformed  spermatozoa 
indicate  want  of  sufficient  vitality  to  push  the  development 
through  to  completion.  The  germ-cells  start  out  apparently 
to  perform  their  functions  normally,  but  later  succumb  to  the 
conflicting  forces  at  work  within  their  boundaries. 

As  to  why  the  reoroductive  organs  should  be  more  sus- 
ceptible to  abnormal  changes  than  other  regions  of  the  body, 
we  have  no  clew.  Darwin  has  pointed  out  repeatedly  the 
curious  parallel  between  crossing  and  the  change  produced  by 
physical  conditions.  Animals  and  plants  removed  from  their 
natural  environment  are  extremely  liable  to  have  their  repro- 

17 


ductive  systems  affected .  Still  he  recognizes  that  sterility  is 
incidental  and  not  a  necessary  concomitant  of  hybridism. 
Hybridization  in  some  forms,  indeed,  increases  fertility. 

SUMMARY. 

i.  The  usual  four  types  of  germinal  cells  are  recognizable, 
viz:  (i)  spermatogonia,  (2)  primary  spermatocytes,  (3)  sec- 
ondary spermatocytes,  and  (4)  spermatids.  Sertoli  or  nurse 
cells  are  likewise  present. 

2.  The  number  of  chromosomes  in  the  spermatogonia  is 
sixteen,  in  primary  spermatocytes  eight,  and  in  secondary 
spermatocytes  four. 

3.  The  spermatogonia  vary  considerably  in  appearance  at 
different  phases  of  their  activity.  A  sphere  (idizome),  within 
which  the  centrosome  lies,  is  visible  before  division. 

4.  A  filament  two  or  three  times  as  long  as  the  other 
chromatic  bodies  in  the  nucleus  is  cast  out  into  the  cyto- 
plasm before  the  formation  of  the  spindle  for  division  of  the 
spermatogonium. 

5.  Synapsis  occurs  in  the  primary  spermatocytes,  during 
which  there  is  a  marked  drifting  of  the  chromatin  to  the  side 
of  the  nucleus  in  contact  with  the  sphere. 

6.  At  the  division  of  the  primary  spermatocyte,  only  eight 
chromosomes  are  present,  but  they  are  in  the  form  of  heavy 
rings,  and  are  evidently  bivalent. 

7.  In  division  the  chromosomes  break  transversely,  and  as 
they  move  apart,  remain  connected  by  threads  of  the  linin  casing 
which  encapsuled  the  chromosomes.  These  threads  form  the 
interzonal  fibers. 

8.  Intermediate  bodies  are  present  at  the  equator  of  the 
interzonal  fibers,  and  mark  out  the  path  of  the  ensuing  division 
of  the  cvtoplasm. 

9.  At  the  division  of  the  secondary  spermatocytes,  the  four 
chromosomes  which  appear  are  of  the  same  size  and  shape  as 
those  of  the  preceding  division. 

10.  In  the  transformation  of  the  spermatid  the  first  per- 
ceptible change  is  in  the  centrosome.  It  divides,  and  one  of 
the  resulting  centrosomes  enlarges  and  becomes  ring-shaped. 
The  axial  filament  of  the  tail  first  appears  as  a  thread  connect- 
ing the  two  centrcsomes,  but  later  continues  backward  through 
the  ring-like  centrosome  and  out  of  the  cell. 

18 


11.  The  smaller  centrosome,  together  with  material  of 
cytoplasmic  origin,  finally  comes  to  lie  inside  of  the  nuclear 
membrane.  Although  a  middle  piece  appears  to  be  absent  in 
the  adult  spermatozoon,  it  seems  probable  that  this  centrosome 
within  the  nucleus  may  function  as  a  middle  piece  which  has 
become  obscured  by  a  covering  of  chromatin. 

12.  The  nucleus  elongates  to  form  the  long  head.  It  con- 
tains a  central  core  of  chromatin  in  the  form  of  a  spiral  filament, 
which  splits  later  to  form  a  double  spiral. 

13.  The  head,  during  the  later  stages  of  development, 
undergoes  a  very  great  contraction,  but  the  spiral  arrangement 
of  the1  chromatin  still  persists  in  a  modified  form. 

14.  The  chromatin  appears  finally  to  be  arranged  within 
the  head  in  a  series  of  vesicles.  A  remarkable  fact  is  that  the 
number  of  vesicles  is  the  same  as  the  reduced  number  of 
univalent  chromosomes  should  be,  namely,  eight. 

15.  The  head-spine  originates  from  a  bubble-like  mass  of 
material  which  arises  in  the  sphere  of  the  spermatid. 

16.  The  general  plan  of  spermatogenesis  in  hybrid  pigeons 
is  not  essentially  different  from  that  of  normal  pigeons.     ' 

17.  All  hybrid  pigeons  exhibit  multipolar  spindles  and 
asymmetrical  distributions  of  the  chromatin  in  cell  division. 
These  irregularities  are  much  more  infrequent  in  fertile  hybrids. 

18.  Infertile  hybrids  show  in  addition  a  deformed  sperma- 
tozoon, and  often  a  marked  degeneration  of  the  germinal  cells. 

19.  The  irregularities  of  division  are  confined  for  the  most 
part  to  the  primary  spermatocytes.  Likewise  it  is  in  these  cells 
that  the  formation  of  bivalent  chromosomes  occurs  normally. 
In  hybrids,  it  would  seem  that  the  conflicting  tendencies  of  the 
two  parental  plasmas  frequently  render  the  union  of  the  single 
chromosomes  to  form  the  double  (bivalent)  types  impossible  or 
abnormal.  There  seems  to  be  an  attempt  on  the  part  of  each 
plasma  to  assert  its  individuality.  This  visible  incompatibility 
of  the  chromosomes  from  widely  different  species  serves  as  an 
index  to  a  kindred  lack  of  harmony  between  the  plasmas  of  more 
nearly  related  forms,  so  that  even  though  pseudo-reduction  does 
occur  and  normal  division  of  the  bivalent  chromosomes  follows, 
the  identity  of  the  individual  species  is  still  retained  through  the 
segregation  of  the  maternal  and  paternal  chromosomes  into  sep- 
arate cells,  which  may  be  considered  "pure"  germ-cells  (con- 
taining qualities  of  only  one  species). 

19 


20.  Union  of  two  cells  containing  characteristics  of  the 
same  species  would  occasion  a  reversion  to  that  species.  Union 
of  two  cells  representing  each  of  the  two  original  species  would 
yield  an  offspring  of  the  mixed  type.  The  latter  would  pre- 
dominate because  of  the  greater  probability  of  such  union. 
Besides,  through  the  mixing  just  indicated,  variability  may  be 
due  also  in  some  cases  to  the  not  infrequent  inequalities  in  the 
division  of  individual  chromosomes,  through  which  varying  pro- 
portions of  the  chromatin  of  each  species  may  appear  in  certain 
of  the  mature  germ-cells. 

21.  Irregular  divisions  can  not  of  themselves  account  en- 
tirelv  for  reversion  and  variations,  because  double  spindles  and 
irregularities  in  the  formation  of  bivalent  chromosomes  are  by 
far  too  few  to  equal  the  percentage  of  reversions  seen  in  such 
mild  crosses  as  the  brown  and  the  white  ring  dove.  One  is 
forced  to  the  conclusion  expressed  above,  that  the  double- 
spindled  and  multipolar  types  of  cells  which  occur  in  hybrids 
between  very  divergent  forms  are  but  exaggerated  images  of 
a  tendency  which  exists  in  the  primary  spermatocytes  of  normal 
appearance,  which  are  to  be  found  in  all  hybrids. 

22.  The  above  conception  may  likewise  afford  a  clew  to 
the  problem  of  why  certain  plants  will  come  true  from  slips  or 
grafts,  but  not  from  seed.  The  explanation  may  be  sought  in 
the  pseudo-reduction  period  of  the  germ-cell. 

Department  of  Biology. 
University  oe  Cincinnati. 
October.  IQ02. 

Note. — Inasmuch  as  the  present  paper  is  a  resume  of  the  writer's 
thesis  of  1900,  it  is  not  deemed  advisable  to  enter  into  a  discussion 
of  any  of  the  more  recent  papers  which  have  a  bearing  on  the 
results  obtained.  Juel's  paper  on  hybrids  of  Syringa,  which  did  not 
appear  until  later,  is  perhaps  the  most  significant.  His  hybrids 
exhibited  abnormal  mitoses  similar  to  those  that  are  found  abund- 
antly in  pronounced  pigeon-hybrids.  From  his  facts  I  would  sug- 
gest that  the  same  interpretation  as  set  forth  in  my  conclusions 
holds  true,  although  it  can  not  be  extended  in  the  same  detail  to 
fertile  forms,  because  his  hybrids  were  sterile.  Two  very  brief  ab- 
stracts of  one  or  two  of  the  more  unusual  points  presented  in  the 
writer's  thesis  have  appeared  in  Science,  but  thus  detached  from  the 
body  of  the  work,  it  would  seem  from  the  tenor  of  letters  which 
have  been  received  that  in  some  cases  a  misunderstanding  of 
the  writer's  exact  position  has  arisen.  It  is  hoped  that  the  present 
paper  will  set  forth  the  main  facts  in  their  true  perspective. 

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